This invention relates to electromechanical impact printing devices of the type employing a rotary type-face-carrier. More particularly, this invention relates to a homing system for printing systems of the kind noted wherein the rotary type-face-carrier is rotated for character selection purposes.
Rotary printing systems are known in which the printhead comprises a rotary type-face-carrier, e.g., a printwheel, including a plurality of resilient pads or fingers bearing printing elements. The printhead is located on a carriage for translation from one print position to the next along a print line direction during printing operations. This operation is performed by translating the carriage from left to right and back, using a series of cables and pulleys driven by a D.C. motor controlled by electronic circuitry. As the carriage-head assembly is moved from a print position to the next along the print line direction, the type-face-carrier is rotated about its axis for character selection purposes. This operation rotates the printwheel until the pad bearing the character to be printed faces the desired print position and is aligned with the striking end of a printhammer also mounted on the carriage. Printing is then performed while the carriage is momentarily stopped by actuating the printhammer to impress the character borne by the pad against an inking ribbon and a print receiving medium. After the printhammer rebounds toward its original rest position, the printwheel is rotated so that the next proper character pad is aligned with the printhammer, while the carriage is translated to the next character print location where the next character is to be printed. This process continues until a complete line has been printed, after which the carriage is returned to the next line starting position, e.g., which could either be the left margin position or the end of the following line in case of bi-directional printing, and the print receiving medium is moved in preparation for the printing of the next line of characters.
Proper operation of such rotary printing systems depends on a number of factors among which the proper character selection is one of the most critical. In so called closed-loop systems, the rotary printwheel cooperates with position sensing devices enabling the system associated logic equipment to determine permanently which character faces the printhammer. For instance, the printwheel includes detection marks or indicia which can be detected by a printwheel home detector during printwheel rotation. In response to detection of the detection marks, a printwheel home signal is generated by the detector and transmitted to the logic circuitry which controls the operation of a motor used to rotate the printwheel. With some additional logic and/or printwheel angular position detection means, the system is kept permanently aware of the printwheel angular situation. Such detection systems are generally located on the translating carriage adding weight thereto.
All of the necessary bulky and expensive sensing equipment used in the closed-loop systems may be avoided provided a few additional logic and control means are added to the machine. In so called open-loop systems, no expensive sensing means is used to permanently track the printwheel angular position and report it to the system control unit. The system needs only to know which character was last printed and which one should be next, to determine through a table look up operation the required printwheel angular displacement to be performed.
Naturally, before printing first starts, i.e., when the power is turned on, and even from time to time during normal printing operation, when a predetermined event occurs, both closed and open loop systems need some kind of homing operation to put the printwheel into a home angular position, i.e., a reference position, known to the system. This operation is fairly simple when dealing with closed loop type of equipment. For instance, the printwheel may be commanded to rotate in a given direction up to a velocity corresponding to the movement of a predetermined number of character element spaces per time unit and then be continuously rotated in that same direction at this velocity until the printwheel home detector detects a home mark on the wheel a predetermined number of times. When this occurs, a signal is generated indicating home position to the system which will then track the wheel rotation through a control of the rotation driving means.
While the requirement for homing operation is obviously even more critical with open loop type of equipment where any error in homing operation would lead to a totally incorrect subsequent printing, no sensing equipment is normally available in such type of equipment, which could be used for homing purposes.
Some open loop printers have been equipped with optoelectronic or with electromagnetic type of sensors especially intended for homing purposes. These devices and their read out electronics are relatively expensive.